LEESBURG, Va. - With the presidential race entering its final full week, most signs point to a comfortable victory for Senator Barack Obama, who is poised to fulfill his quest to redraw the electoral map by picking off a handful of historically Republican states.

He's not quite measuring drapes for the White House, but there is an unmistakable sense among Obama's aides, many supporters, pundits, and people around the nation that, barring something dramatic, he will be the 44th president of the United States and its first African-American one.

But what has this presidential campaign been if not unpredictable? Republican John McCain was left for dead last year only to come roaring back to seize the GOP nomination. A candidate named Clinton actually lost on the national stage. Hockey moms, kung fu stars, talking polar bears, and plumbers have all made unexpected cameos.

And significant variables remain. Will latent racial bias quietly peel whites away from Obama on Election Day? Will the millions of newly engaged, newly registered voters actually cast ballots in big numbers? Will the small-but-influential group of undecided voters in key states break against Obama, as many did in the primaries? Will independents shift to McCain to avoid a Democratic monopoly in Washington?

Are the flock of polls that show Obama well ahead simply wrong, as polls sometimes are, and those that show a close or narrowing race on the mark?

"A lot of things could change," said George Vaughn, a 58-year-old Obama supporter in Indianapolis who works in transportation logistics. "You just don't know. I'm excited. But I'm also a realist."

Indeed, even as Obama, his campaign team, and his enthusiasts can smell victory, most are unwilling to give in to elation, fearful that Democrats will get burned again by some unknown cocktail of attack ads, errant polling, national security threats, and balloting problems on Nov. 4.

"I feel like we got a righteous wind at our backs here," Obama told 35,000 people in Leesburg on Wednesday, a noteworthy crowd in a state that Democrats have not won since 1964. "But we're going to have to work. We're going to have to struggle. We're going to have to fight" until the polls close.

The night before, Obama's wife, Michelle, warned supporters in Miami to ignore all the predictions of an easy win.

"We can take nothing for granted," she said. "My view is that Barack Obama is the underdog and will continue to be the underdog until he's sitting in the Oval Office. We have to act like he's 20 points behind."

Overall, the trend lines for Obama are good, both on the ground and in the national mood. Polls show him leading McCain by significant margins in battleground states from Virginia to Colorado. Democrats enjoy sizable advantages in voter registration in key states such as Florida and Pennsylvania. Obama is benefiting from a surge in Democrats voting early. And surveys show that record numbers of Americans believe the country is headed in the wrong direction.

Some even invoke the landslides of Democrat Lyndon Johnson in 1964 and Republican Richard Nixon in 1972.

"We're looking at Nixon over McGovern poll numbers. We're looking at Johnson over Goldwater," said presidential historian Douglas Brinkley of Rice University. "We're looking at very high numbers for Barack Obama right now, which spell decisive victory."

But even though candidates with this big a lead traditionally win, Brinkley said, it is too soon to bet on it.

"It's a very volatile week, this one right now," he said. "It's when campaigns throw out all of their last-minute tricks and ploys."

But with his huge edge in campaign money, Obama seems well positioned to respond to any last-minute attacks; he has already bought a half-hour of prime time Wednesday night to make his closing argument to voters.

Still, there are a few precedents and forces that could work against Obama in the closing days of the campaign.

One that gets a lot of attention is the so-called Bradley effect - an unproven theory named for Tom Bradley, a black Los Angeles mayor who lost his 1982 gubernatorial bid despite polls suggesting that he would win handily - which holds that surveys can hide prejudices against black candidates.

It is impossible to know whether this will have any bearing on Obama's candidacy, but an Associated Press-Yahoo News poll last month suggested that racial bias could cost Obama as much as 6 percentage points, and some supporters cite it as a concern.

"We just pray that the country can get past those kinds of things when we get in the voting booth," said Vaughn.

Another question is whether turnout will match the expectations of pollsters, some of whom are basing their results on an unprecedented disparity between the number of Democrats and Republicans who they predict will cast ballots.

Most recent national polls show Obama with a lead between 5 and 15 percentage points, but a few last week showed a dead heat. There is no clear explanation for the difference.

In the Democrats' weekly radio message, Michelle Obama urged supporters yesterday to make sure to vote, saying, "We can't look back and think about what might have been."

A big variable in all this are the undecided voters, who average about 6 percent of the electorate in national polls in the month of October and who form a pivotal bloc in close battleground states such as Florida and Indiana.

Many undecided voters are white women, according to a recent Pew poll. (With so many people voting early this year, though, there may be a smaller pool of undecided voters come Election Day.)

It is difficult to extrapolate too much from the primaries, but throughout the winter and spring, exit polling indicated that Obama had trouble closing the deal with undecided voters.

Obama's campaign manager, David Plouffe, said Friday that he did not consider the exit polls from the primaries indicative on that point, and that Obama's better favorable-unfavorable ratings among voters than McCain will help the Illinois senator win many late deciders.

"I'm sure Senator McCain will get his share of them," Plouffe told reporters. "But the notion that somehow these late undecideds are going to break in a disproportionate way for Senator McCain, we just don't see that."

One trend working in Obama's favor is that, when there is such dissatisfaction with the direction of the country, voters who make up their minds late tend to go with the challenger, pollsters say. McCain's Republican Party has occupied the White House for eight years.

Interviews last week with Obama supporters in the key battlegrounds of Florida, Virginia, and Indiana found mixed emotions as voters look ahead to Nov. 4.

Debbie Davidson of Indianapolis, who works for a nonprofit on HIV/AIDS, fears Republican malfeasance at polling places, saying, "They don't care how they win."

"I feel very hopeful, but also very cautious," said Davidson, 50. "Scared."

Those mirror the sentiments of Jen Halpin of Leesburg, a 46-year-old marketing director for a fitness center. "I want it to be over now," she said. "Just end it!"

Other Obama supporters are more sanguine.

Barbara Anderson, a 66-year-old retired teacher from Richmond, said she marched with Martin Luther King Jr. in Richmond and Petersburg, Va. more than 40 years ago. She said she never thought she would see an African-American on the cusp of the presidency.

"I am living his dream," she said. "And I am so blessed."

But is she nervous? "Not at all," Anderson said. "I just feel it. I really do."

For their part, some McCain supporters believe the race can still shift.

Ed Alf, a 73-year-old retiree from Cincinnati, said he expects voters will move to the Arizona senator once they finally understand "where Obama comes from, what his values are, what his background is."

"I'm sure when voters that made up their minds early are getting the real facts, I think they'll change," Alf said. "A lot of it will come out in the last week."

Sasha Issenberg of the Globe staff contributed to this report; Scott Helman can be reached at shelman@globe.com.